Spark advance mechanism for solid state ignition systems

ABSTRACT

A spark advance mechanism for an internal combustion engine is included in a variable reluctance voltage generator comprised of a magnet embedded in a member rotating in synchronism with the engine which moves the magnet passed a sensor coil having a shaped core. When the voltage generated in the sensor coil by the rate of change of flux from the magnet rises to a given threshold amplitude it triggers an ignition circuit thereby producing a spark to ignite fuel in the engine. Spark advance is accomplished by varying the rate of change of flux through selectively shaping the portion of the core which is adjacent to the path of the magnet.

lnventors Roland J. Foreman Franklin Park; William J. Warner,Schaumburg, both 01, Ill.

Appl. No. 828,990

Filed May 29, 1969 Patented Aug. 17, I971 Assignee Motorola, Inc.

Franklin Park, Ill.

SPARK ADVANCE MECHANISM FOR SOLID AC, 148 15,149,149 D, 310/159 [56]References Cited UNITED STATES PATENTS 2,446,671 8/1948 Short et a1.123/148 E 3,277,875 10/1966 Miki 123/148 E 3,447,521 6/1969 Pifeo r123/148 E 3,465,739 9/1969 Burson 123/148 E X 3,490,426 1/1970 Farr123/148 E X Primary Examiner--Laurence M. Goodridge Attorney-Mueller &Aichele ABSTRACT: A spark advance mechanism for an internal combustionengine is included in a variable reluctance voltage generator comprisedof a magnet embedded in a member rotating in synchronism with the enginewhich moves the magnet passed a sensor coil having a shaped core. Whenthe voltage generated in the sensor coil by the rate of change of fluxfrom the magnet rises to a given threshold amplitude it triggers anignition circuit thereby producing a spark to ignite fuel in the engine.Spark advance is accomplished by varying the rate of change of fluxthrough selectively shaping the portion of the core which is adjacent tothe path ofthe magnet.

PATENIED AUG] 7 I97l CRANK ANGLE DEGREES BEFORE TDCv R. P. M. INTHOUSANDS INVENTORS ROLAND J. FOREMAN WILUAM J. WARNER ATTYS.

BACKGROUND OF THE INVENTION Breakerless capacitor discharge ignitionsystems utilizing solid state devices in place of prior art mechanicalbreaker points have been proposed for use in many types of internalcombustion engines. Contact burning and timing drift which plague theprior art systems are representative of the problems eliminated by thesebreakerless systems. Electronic spark advance mechanisms with no movingparts in contact with each other have also been designed to furtherimprove the breakerless ignition systems by replacing mechanical sparkdevices.

One of these electronic spark advance mechanisms is built into avariable reluctance voltage generator wherein a shaped reluctancesegment, fastened to and rotated with the flywheel of an internalcombustion engine, shifts the rate of change of magnetic flux in amagnetic pickup thereby generating a voltage pulse in coincidence witheach revolution of the flywheel. The rotational positions of theflywheel when each of these pulses occur varies as a function of theangular velocity of the flywheel because of the spaced reluctancesegment. When each of these subsequent voltage pulses reaches athreshold or trigger amplitude, it activates circuit elements in anignition circuit which discharge an ignition capacitor through anignition transformer thus providing a high tension igniting spark in aspark plug.

The magnetic pickup of the generator is comprised ofa permanent magnetfixed in abutting relation to a core which is part of a flux pathincluding an airgap through which the spaced segment is moved. Alsoincluded in the magnetic pickup is a coil electrically coupled to theflux so as to develop the trigger or voltage pulses in response to therate of change thereof.

Problems inherent in the construction of this magnetic pickup, however,reduce its reliability and the adaptability of the spark advancemechanism. One particular problem relates to mechanically fixing themagnet to the core of the coil. Sometimes the magnet is held to the coreby a clip or by an adhesive either of which tends to weaken with age andheat. The magnet, consequently, can be vibrated loose thereby reducingthe Iongivity and reliability of the ignition system. Furthermore, whiledesigning or improving the ignition system it is often desirably tomodify the spark advance characteristic of the generator. In theforegoing prior art embodiment it may be necessary to disassemble theengine and remachine the flywheel and the shaped reluctance segment toaccomplish the desired modification.

SUMMARY OF THE INVENTION An object of one embodiment of the invention isto provide an improved mechanism for electronically advancing ignitionsparks for use by an ignition system.

Another object is to provide an ignition system having a spark advancemechanism which reliable, easy to install, and contains a minimum numberof parts.

The improved spark advance mechanism of one embodiment of the inventionis included in a variable reluctance voltage generator which suppliespulses to trigger a capacitor discharge ignition circuit. The componentsof the ignition circuit respond at a threshold amplitude of each pulseto discharge an ignition capacitor through an ignition transformerconnected to a spark-producing device for providing a spark whichignites fuel in the engine. This voltage generator includes a magnetfastened to a rotatable member synchronized with the engine and a sensorcoil wound on a core of low reluctance material having a shaped portionadjacent to the path of the magnet. The changing flux from the magnetflowing through the core produces a trigger pulse in the sensor coilwhich has an amplitude proportional to the rate of change of flux. Asthe magnet passes the core the shaped portion thereof provides a gapbetween it and the magnet which varies at a rate depending on the speedof the magnet thus changing the reluctance and magnetic flux through thecore at a predetermined rate for a given angular velocity of therotatable member. The rate of change of magnetic flux is, consequently,a function of both the angular velocity of the rotating member and theshape of the gap. The amplitude of each trigger pulse, therefore, risesto the firing amplitude at a rota- 'tional position of the magnet whichvaries with the angular velocity of the rotatable member and the shapeof the core to produce spark advance. The placement and shape of thecore can be conveniently tailored to provide different spark advanceversus r.p.m. characteristics for different engines and operatingconditions. Since the spark advance mechanism of the preferredembodiment of the invention does not include the magnetic pickup andreluctance segment of the prior art, the problems associated therewithare eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a plan view of the variablereluctance generator of one embodiment of the invention;

FIG. 2 illustrates the waveform of the voltage induced by the changingmagnetic flux in the charge and sensor coils;

FIG. 3 is a schematic diagram of a capacitor discharge ignition circuit;

FIG. 4 is a graph illustrating spark advance as a function of enginer.p.m.; and

FIG. 5 shows an outline ofa core for the sensor coil used in oneapplication.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, thevariable reluctance voltage generator includes member 10 which is theflywheel or some other member rotating in synchronism with the internalcombustion engine. Magnet 12 is embedded in or fastened to member l0 andmoved therewith. Charge coil 14 and its pole piece 16 are positioned ina spaced relation to the path of magnet 12. Moreover, core 18 of sensorcoil 20 is positioned in a spaced relation to pole piece 16 and the pathof magnet 12. As magnet 12 is rotated in a counterclockwise directionpast pole piece 16, the changing flux from the magnet passing throughpole piece 16 induces an alternating voltage across charge coil 14having the general shape of the waveform in FIG. 2. The magnet thenproceeds to pass shaped core 18 to likewise induce a voltage of theshape shown in FIG. 2 across coil 20.

It should be noted that the flux path for core 18 includes the adjacentleg 21 of pole piece 16. The advantage of this structural arrangementwill be subsequently pointed out.

In FIG. 3 charge coil 14 and sensor coil 20 are shown connected toignition circuit 21. Charge coil I4 is connected between ground or areference potential, and a rectifying diode 22 is connected thereto. Thediode 22 is poled to allow the positive pulse 23 (FIG. 2) of the voltageinduced in coil 14 to charge ignition capacitor 24. The capacitor 24 isconnected to anode 26 of silicon-controlled rectifier (SCR) 28. Sensorcoil 20 is connected across gate 30 and cathode 32 of SCR 28. The seriescombination of diode 34 and resistor 36 is connected in parallel withsensor coil 20 and gate cathode junction by of SCR 28. Diode 34 protectsthe gate cathode junction by conducting on the negative portions 37 and38, shown in FIG. 2, of the waveform induced across sensor coil 20.Resistor 36 limits the current flow through the circuit comprised ofcoil 20, diode 34, and resistor 36 thus reducing hysteresis orsaturation recovery problems in core 13 caused by voltage having largeamplitudes which are generated at high r.p.m. of member 10.

Thermistor 39, connected in parallel with sensor coil 20, has a negativetemperature coefficient. It temperature compensates for the loweringthreshold or firing voltage of SCR 28 at high temperature by decreasingits resistance to the induced potential applied to the gate 30 therebyrequiring correspondingly more voltage to tire the SCR. In addition, itcompensates for the raising threshold voltage at lowering temperature byincreasing its resistance to the induced voltage in coil 20, therebyrequiring correspondingly less voltage to fire the SCR 28. The overallresult, consequently, is that the firing amplitude of the triggervoltage across sensor coil 20 remains constant with temperature eventhough the characteristics of SCR 28 are changing.

In operation as magnet 12 passes charge coil 14, a positive voltage isdeveloped across ignition capacitor 24. An instant of time later asmagnet 12 passes core 18 of sensor coil 20, a positive voltage begins tobuild up between the gate and cathode of SCR 28. When the positivevoltage reaches the threshold or firing potential, the SCR conducts todischarge ignition capacitor 24 through the primary winding 40 ofignition transformer 42 thereby producing a high tension voltage insecondary winding 44 which appears across spark gap 46 of spark plug 47to ignite fuel in the internal combustion engine.

For the internal combustion engine to operate at maximum efficiency, itis necessary for each ignition spark to occur at a variable period oftime before the piston reaches top dead center (TDC). An ignition sparkoccurring before TDC is said to be advanced and the amount of advance ismeasured in terms of crank angle degrees before TDC. The optimum amountof spark advance for best power and minimum fuel depends on manyconditions but in general it should increase as the engine r.p.m.increases.

The spark advancing function of shaped core 18 which is made of ferriteor some other low reluctance material will now be explained. Referringback to FIG. 1, as magnet 12 approaches core 18 it reaches a point onits path where the magnetic flux therefrom encounters a gap 48 initiallyhaving the width of dimension A which diminishes to a width of dimensionB. This gap deviates, therefore, at a predetermined rate for a givenrate of rotation of member 10. Since the flux through core 18 varies asthe reluctance controlled by the gap width, the rate of change of fluxwill correspondingly vary as determined by the changing gap width for afixed angular velocity of member 10. The rate of change of flux alsovaries with the speed of magnet 12 which is controlled by the angularvelocity and the diameter of member 10.

The amplitude of the voltage produced across sensor coil 20 isproportional to the rate of change of flux. Consequently, at a slowangular velocity of member 10, the rate of change of flux from magnet 12might not be great enough to produce the firing amplitude across sensorcoil 20 until a point in time when pole 52 of magnet 12 approaches tip54 of the shaped core 18. Alternatively, when member 10 is rotating at ahigh angular velocity the rate of change of flux, because of theincrease in speed of magnet 12, will produce the firing amplitude at apoint in time when the pole 52 of magnet 12 is approaching the leadingedge 56 of shaped core 18. As a result, the amplitude of the triggerpulse in the sensor coil rises to the firing amplitude at the rotationalposition of magnet 12 which varies in accordance with the angularvelocity of member 10 thus producing the desired spark advance.

By placing core 18 in a spaced relation to leg 21 on pole piece 16, thetotal reluctance in the flux path for sensor coil 20 is reduced thusenabling the change in reluctance across vary ing gap 48 to be a greaterproportion of the total reluctance and thereby increasing thesensitivity of the spark advance mechanism. Pole piece 16, therefore,provides most of the flux path for charge coil 14 and a portion of theflux path for sensor coil 20. Moreover, by utilizing pole piece 16 inthis manner less overall weight is added to the generator than would bethe case if an additional flux return leg was built into core 18.

The combination of sensor coil 20 and its core 18 replace the magneticpickup units employed in the prior art, which used a variable reluctancesegment in place of magnet 12. Reluctance gap 48 of the preferredembodiment of the invention can be easily modified by changing the shapeof core 18 adjacent flywheel 10. To modify the prior art variablereluctance generator, however, it is usually necessary to remove theflywheel from the engine and the shaped segment from the flywheel andreform both the flywheel and the segment. In addition magnet 12 of thepreferred embodiment is embedded in the material of the flywheel ratherthan being attached to a flux core where it would be subject tovibration which might shake it loose thereby rendering the ignitionsystem inoperative.

One particular capacitor discharge ignition system for an internalcombustion engine of a chain saw utilizing the spark advance mechanismof the preferred embodiment of the invention has the followingdimensions: (See FIG. 1 for the placement ofcore 18 with respect to theflywheel 10 and pole piece 16.) A=0.l53 inch B=0.010 inch C =l.200 inchFIG. 5 shows the actual size and shape of the ferrite core 18 for sparkadvance mechanism of the ignition system.

The curve of FIG. 4 shows the resulting spark advance characteristic 60in terms of crank angle degrees before TDC as a function of rpm. inthousands. For the particular chain saw engine and operating conditionsit is desirable for the spark advance to rapidly increase from 300r.p.m. to 1,000 r.p.m., as shown between points 62 and 64 on FIG. 4.Less spark advance is required and thus provided above 1,000 r.p.m. Theshape and placement of core 18 was adjusted to give the desiredcharacteristic. The scope of the invention includes tailoring the shapeand placement of core 18 to give other spark advance characteristics.

What has been described, therefore, is a simple spark advance mechanismwhich is reliable, easy to maintain, and inexpensive. The mechanism hasno moving parts in contact with each other and it can be convenientlymodified to produce specific spark advance characteristics by changingthe shape and spacing of the core of the sensing coil.

We claim:

l. A capacitor discharge ignition system including a pulseactuatedcircuit for discharging the capacitor to fire the engine, including incombination:

pulse generator means having a rotating member, magnet means providingflux and being integral with said rotating member, said magnet meansbeing moved by said rotating member along a predetermined path;

a first pole piece having first and second leg portions each positionedin a spaced relation to said predetermined path of said magnet means acharging coil wound about one of said leg portions whereby said magnetmeans passing by said pole piece completes a flux path through saidmagnet and said first and second leg portions of said pole piece therebyinducing a current in said charging coil for charging the ignitioncapacitor subsequent to the discharge thereof;

a second pole piece having a first portion positioned in a spacedrelation to said predetermined path of said magnet means and a secondportion extending in a spaced relation to said second leg portion ofsaid first pole piece, a pulse producing triggering coil wound aboutsaid second pole piece;

said magnet means further completing a flux path through said second legportion of said first pole piece, said extended second portion of saidpole piece and said second pole piece to generate a pulse in saidtriggering coil for discharging the ignition capacitor.

2. The capacitor discharge ignition system of claim 1 wherein thepulse-actuated circuit is responsive to said pulse reaching a thresholdlevel to discharge the ignition capacitor to produce a spark forigniting fuel in the engine, and said first portion of said second polepiece has a selectively shaped portion positioned in s spaced relationto said predetermined path of said magnet means and producing a changinggap therebetween which varies at a rate responsive to the angularvelocity of the rotating member, said changing gap varying the rate ofchange of magnetic flux from said magnet means as said magnet meanspasses said shaped portion to produce said pulse in said pulse-producingtriggering coil which rises to said threshold level at a rotationalposition of said rotating member which varies in accordance with theangular velocity thereof thus providing a selected spark advancecharacteristic.

3. The electronic ignition system of claim 2 wherein said selectivelyshaped portion of said second pole piece provides a predetermineddecreasing gap between it and said magnet means as said magnet meanspasses said shaped portion so that the degree of rotation of saidrotating member at which said pulses reach said threshold level varieswith the angular velocity of said magnet means to provide said selectedspark advance characteristic 4. The capacitor discharge ignition systemof claim ll wherein said rotating member is the flywheel of an internalcombustion engine.

5. The capacitor discharge ignition system of claim 1 wherein the pulseactuated circuit is comprised of:

trigger means having input, control and output electrodes;

said charging coil being connected to the ignition capacitor forcharging the same; the ignition capacitor being connected to said inputelectrode of said trigger means; said pulse producing trigger coil beingconnected to said control electrode of said trigger means for operatingthe same, said trigger means being rendered conductive in response toeach of said pulses to discharge said ignition capacitor to produce thesparks for igniting fuel in the engine.

6. The capacitor discharge ignition system of claim 5 wherein saidtrigger means is a semiconductor device having a trigger threshold levelwhich changes with temperature deviation; temperature variableresistance means connected to said control electrode of saidsemiconductor device, said temperature variable resistive means having aresistance which changes with said temperature deviation to vary theamplitude of said pulses to compensate for said changes in said triggerthreshold level so that said ignition system provides a spark advancecharacteristic which is substantially independent of said temperaturedeviation.

1. A capacitor discharge ignition system including a pulseactuatedcircuit for discharging the capacitor to fire the engine, including incombination: pulse generator means having a rotating member, magnetmeans providing flux and being integral with said rotating member, saidmagnet means being moved by said rotating member along a predeterminedpath; a first pole piece having first and second leg portions eachpositioned in a spaced relation to said predetermined path of saidmagnet means a charging coil wound about one of said leg portionswhereby said magnet means passing by said pole piece completes a fluxpath through said magnet and said first and second leg portions of saidpole piece thereby inDucing a current in said charging coil for chargingthe ignition capacitor subsequent to the discharge thereof; a secondpole piece having a first portion positioned in a spaced relation tosaid predetermined path of said magnet means and a second portionextending in a spaced relation to said second leg portion of said firstpole piece, a pulse producing triggering coil wound about said secondpole piece; said magnet means further completing a flux path throughsaid second leg portion of said first pole piece, said extended secondportion of said pole piece and said second pole piece to generate apulse in said triggering coil for discharging the ignition capacitor. 2.The capacitor discharge ignition system of claim 1 wherein thepulse-actuated circuit is responsive to said pulse reaching a thresholdlevel to discharge the ignition capacitor to produce a spark forigniting fuel in the engine, and said first portion of said second polepiece has a selectively shaped portion positioned in s spaced relationto said predetermined path of said magnet means and producing a changinggap therebetween which varies at a rate responsive to the angularvelocity of the rotating member, said changing gap varying the rate ofchange of magnetic flux from said magnet means as said magnet meanspasses said shaped portion to produce said pulse in said pulse-producingtriggering coil which rises to said threshold level at a rotationalposition of said rotating member which varies in accordance with theangular velocity thereof thus providing a selected spark advancecharacteristic.
 3. The electronic ignition system of claim 2 whereinsaid selectively shaped portion of said second pole piece provides apredetermined decreasing gap between it and said magnet means as saidmagnet means passes said shaped portion so that the degree of rotationof said rotating member at which said pulses reach said threshold levelvaries with the angular velocity of said magnet means to provide saidselected spark advance characteristic.
 4. The capacitor dischargeignition system of claim 1 wherein said rotating member is the flywheelof an internal combustion engine.
 5. The capacitor discharge ignitionsystem of claim 1 wherein the pulse actuated circuit is comprised of:trigger means having input, control and output electrodes; said chargingcoil being connected to the ignition capacitor for charging the same;the ignition capacitor being connected to said input electrode of saidtrigger means; said pulse producing trigger coil being connected to saidcontrol electrode of said trigger means for operating the same, saidtrigger means being rendered conductive in response to each of saidpulses to discharge said ignition capacitor to produce the sparks forigniting fuel in the engine.
 6. The capacitor discharge ignition systemof claim 5 wherein said trigger means is a semiconductor device having atrigger threshold level which changes with temperature deviation;temperature variable resistance means connected to said controlelectrode of said semiconductor device, said temperature variableresistive means having a resistance which changes with said temperaturedeviation to vary the amplitude of said pulses to compensate for saidchanges in said trigger threshold level so that said ignition systemprovides a spark advance characteristic which is substantiallyindependent of said temperature deviation.